Adsorption of sulfate ion from water by zirconium oxide-modified biochar derived from pomelo peel

Journal Pre-proofs Adsorption of Sulfate Ion from Water by Zirconium Oxide-Modified Biochar Derived from Pomelo Peel Hanting Ao, Wei Cao, Yixia Hong, Jun Wu, Lin Wei PII: DOI: Reference:

S0048-9697(19)35084-3 https://doi.org/10.1016/j.scitotenv.2019.135092 STOTEN 135092

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Science of the Total Environment

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4 July 2019 15 October 2019 19 October 2019

Please cite this article as: H. Ao, W. Cao, Y. Hong, J. Wu, L. Wei, Adsorption of Sulfate Ion from Water by Zirconium Oxide-Modified Biochar Derived from Pomelo Peel, Science of the Total Environment (2019), doi: https://doi.org/ 10.1016/j.scitotenv.2019.135092

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Adsorption of Sulfate Ion from Water by Zirconium Oxide-Modified Biochar Derived from Pomelo Peel Hanting Ao, Wei Cao*, Yixia Hong, Jun Wu, Lin Wei (College of Civil Engineering, Huaqiao University, Xiamen 361021, China)

*Corresponding author: [email protected]

Abstract Zirconium oxide-modified pomelo peel biochar (ZrBC) was synthesized for the adsorption of sulfate ion from aqueous solution. Zirconyl chloride octahydrate (ZCO) was used to modify pomelo peel biochar into ZrBC. The optimal dose of ZCO for modification is 0.5 mol/L, at which ZrBC shows the highest adsorption of sulfate ion. The adsorbents were characterized by the field emission scanning electron microscopy, X-ray photoelectron spectroscopy and surface area measurement. The results confirm that the presence of zirconium oxides and hydroxide groups on the ZrBC surface, and ZrBC has a porous structure and a higher specific surface area in comparison with pomelo peel biochar. ZrBC shows good affinity for sulfate ion with a maximum sulfate adsorption capacity of 35.21 mg/g, which is much higher than that of pomelo peel biochar (1.02 mg/g). The adsorption of sulfate on ZrBC is pH dependent, and acidic conditions favor the adsorption. The adsorption can reach near-equilibrium in approximately 120 min. The adsorption kinetics and isotherm follow the pseudo second-order equation and Langmuir adsorption model, respectively. Furthermore, nitrate and fluoride anions exhibit little influence on the adsorption of sulfate by ZrBC, whereas phosphate inhibits the adsorption under the same concentration conditions. ZrBC has the potential to be used for removal of sulfate from aqueous solution. Key words: sulfate ion, zirconium oxide, pomelo peel biochar, XPS characterization, adsorption performance.

1 Introduction Sulfate ions is widely distributed in surface water, groundwater and industrial effluents, such as acid mining drainage and pharmaceutical, printing and dyeing wastewater. The main sources of sulfate in natural water are chemical weathering and oxidation processes of sulfur-containing minerals (Fernando et

al., 2018). Although sulfate ion often is considered to be nontoxic, its potential harm to living organisms and environment cannot be ignored. High concentrations of sulfate ion in water can lead to an imbalance in the natural sulfur cycle in ecosystem, and endanger human health under long-term ingestion (Amaral Filho et al., 2016; Pol et al., 1998). Therefore, it is necessary to remove sulfate ion from wastewater before discharge into the surrounding environment. The established methods for sulfate removal from water include chemical precipitation, ion exchange, biological treatment, adsorption and membrane filtration (Gupta et al., 2012; Hong et al., 2014; Silva et al., 2002; Silva et al., 2010). Adsorption separation is widely used because of its rapid and effective removal of sulfate ion. The adsorbent plays an important role in determining the effectiveness of adsorption technology, and the desired adsorbent needs to have a low cost, a high adsorption capacity and be renewable. Granular activated carbon (GAC) and anion exchange resins have been widely studied to remove sulfate from industrial wastewater (Fernando et al., 2018; Haghsheno et al., 2009). However, GAC has low adsorption efficiency, while anion exchange resins are too expensive to be utilized in wastewater treatment. Hence, a cost-effective sulfate adsorption material must be developed. Zirconium oxide particles, which are nontoxic and stable in water, have been used to separate sulfate ion from industrial brine water owing to their high affinity for sulfate (Cui et al., 2012). However, their practical application is limited because zirconium oxides are difficult to immobilize, separate and recycle (Yu et al., 2018). To overcome these drawbacks, researchers have attempted to load zirconium oxides on porous materials such as cellulose, activated carbon, and zeolite (Mulinari and da Silva, 2008). Recently, biochar has attracted great attention as an alternative desirable adsorbent and catalyst support due to its porous structure, low cost and environmental compatibility (Jung and Ahn, 2016). Biochar is a carbon-rich material obtained from the thermochemical or hydrothermal conversion of

biomass under oxygen-limited conditions (Jin et al., 2017; Nielsen et al., 2014). It is believed that biochar has the potential to delay global climate change and to improve soil fertility and water retention (Jin et al., 2017; Joseph et al., 2015; Spokas et al., 2009; Warnock et al., 2007; Yanai et al., 2007). Several reports have confirmed that biochar can serve as an adsorbent to immobilize organic pollutants (Inyang et al., 2015a; Rajapaksha et al., 2014) and heavy metals (Inyang et al., 2015b; Yang and Jiang, 2014) in soil and water. Biochar is usually prepared from agricultural and forestry wastes, such as wood, crop straw and fruit husks, and organic wastes produced in industrial applications and urban life. Pomelo (Citrus maximal) peel is a kind of low-cost biomass waste, and has a sponge-like structure, which is suitable for preparing porous biochar (Argun et al., 2014; Tasaso, 2014). However, ordinary biochar has shown poor adsorption capacity toward anionic species due to its negative surface charge (Bian et al., 2014; Kameyama et al., 2016). Therefore, metal oxides such as magnesia, lanthana, and iron oxides are applied to modify biochar to improve its adsorption capacity for nitrate and phosphate (Wang et al., 2016; Zhang et al., 2012). However, few studies have investigated sulfate adsorption by zirconium oxide-modified biochar. Herein, this study aims to prepare zirconium oxide-modified pomelo peel biochar (ZrBC) and to evaluate its potential to remove sulfate ion from water. Pomelo peel (PP) was selected to generate biochar due to its sponge-like structure and high yield, especially in the main production area, such as Fujian, China. Zirconyl chloride was used to modify the pomelo peel biochar. The generated adsorbents were characterized by field emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (FE-SEM/EDS), N2 adsorption-desorption isotherms and X-ray photoelectron spectroscopy (XPS). Additionally, the adsorption isotherms and kinetics, as well as the effect of environmental factors (i.e., pH and competitive ions) on the sulfate adsorption capacity of ZrBC, were investigated. 2 Materials and methods

2.1 Materials and chemicals Fresh PP was collected from Xiamen, Fujian Province, China. It was washed with distilled water several times and dried at 50°C for 48 h. Chemicals used in this work such as ammonia (NH3H2O), potassium sulfate (K2SO4), potassium nitrate (KNO3), potassium phosphate monobasic (KH2PO4), sodium fluoride (NaF), sodium hydroxide (NaOH) and concentrated hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Zirconyl chloride octahydrate (ZCO, ZrOCl28H2O) was obtained from Aladdin Co., Ltd., China. Deionized (DI) water (18.2 MΩ.cm) was used to prepare experimental solutions. 2.2 Preparation of ZrBC The dried PP was cut up with a knife to obtain particles with a diameter of 0.2~0.3cm, and heated at 10 °C/min up to 400 °C under N2 flow for 1.0 h in a tube furnace (NBD-O1200-50IT, Nobody Materials Science and Technology Co.,Ltd, China). After carbonization, the sample was rinsed with deionized water several times until the pH of filtrate did not change, and then it was dried in an oven at 80 °C overnight. The resulting material was blank pomelo peel biochar, labeled as PBC. A solution of ZCO (0.5 mol/L) of 100 mL was prepared in a 250 mL Erlenmeyer flask. PBC (1.0 g) was

mixed with the ZCO solution in

the flask, which was placed in a shaker for 2.0 h. Ammonia solution (1:3, v/v) was added dropwise into the mixture under stirring to adjust pH value to approximately 4.5. After reaction, the sample was aged at room temperature (25 °C ) for 24 h. Finally, the material was collected by filtration, washed with deionized water until the conductivity of filtrate was less than 2.0 μS/cm, and then dried in a vacuum oven at 50 °C for 12 h. The obtained sample, ZrBC, was sealed in a container. To study the effect of the initial ZCO concentration on the modification process, different ZrBCs were prepared by varying the concentration from 0.25 mol/L to 0.75 mol/L.

2.3 Characterizations The surface morphology and microstructure of PP, PBC and ZrBC were characterized by FE-SEM/EDS (ZEISS Sigm, Carl Zeiss, Germany), which is applied to determine the elemental composition of materials The surface functional groups of the materials were probed by using XPS (Escalab 250Xi, Thermo Fisher Scientific, USA). The monochromatized X-ray source was an Al K-alpha radiation (anode target). The surface area of the adsorbents was measured by N2 adsorption-desorption isotherms at 77 K using a NOVA 2000e surface area analyzer (Quantachrome, USA). The specific surface area and average pore radius were calculated based on BET (Brunauer-Emmett-Teller) method. The total pore volume was analyzed via single-point adsorption at a relative pressure of 0.99. The pore distribution was determined from the adsorption curve by using BJH (Barret–Joyner–Halenda) method. 2.4 Batch adsorption experiments Batch adsorption experiments were performed to study the sulfate adsorption performance on the ZrBC material. A stock sulfate solution with a concentration of 1000 mg/L was prepared by dissolving potassium sulfate (K2SO4) in deionized water. All sulfate solutions used in adsorption experiments were diluted from this stock solution. A certain weight (0.1 g) of the adsorbent (PBC or ZrBC) was added into a 150 mL conical flask containing 50 mL sulfate solution. The flasks were placed in a shaker at 150 rpm and 298 K. After remaining in contact for 24 h, the solid adsorbent and solution were separated by filtration, and the filtrate was collected to detect sulfate concentration. The sulfate ion concentrations in aqueous solution were measured using ion chromatography method (Metrohm, Switzerland). The detection limit of the ion chromatography method is 0.13 mg/L and the relative standard deviation is 1.25%. All adsorptoin experimental samples were designed in duplicate. To study the effect of pH on sulfate adsorption, adsorption experiments were conducted under

different pH conditions. The initial pH of the sulfate solution was changed from 2.0 to 12.0 by using 1.0 mol/L HCl and 1.0 mol/L NaOH. The adsorption isotherms were measured by varying the initial sulfate concentration from 5 to 300 mg/L at the optimum pH. The equilibrium adsorption amount (qe) was calculated by equation (1).

qe 

(C0  Ce )  V , m

(1)

where C0 (mg/L) and Ce (mg/L) are the initial and equilibrium concentrations of sulfate ion, respectively; m (g) is the mass of adsorbent; and V (L) is the volume of sulfate solution. For the adsorption kinetics study, the contact time intervals were 5, 10, 30, 60, 120, 240, 480, and 720 min. In addition, the adsorption kinetics were measured at 288K, 298K, 308K and 318K to investigate the effect of temperature on the adsorption. The effects of coexisting ions (NO3-, PO43-, and F-) on the adsorption of sulfate on ZrBC were also studied in a binary adsorption system, and the concentrations of both sulfate and coexisting ions were 1.0 mmol/L. The concentrations of these ions were also determined by using ion chromatography method (Metrohm, Switzerland). 3 Results and discussion 3.1 Effect of initial ZCO concentration on the preparation of ZrBC As shown in Fig. 1, all the biochars modified with different initial ZCO concentrations exhibit sulfate removal ability. The sulfate adsorption amount on ZrBC prepared with an initial ZCO concentration of 0.5 mol/L is higher than that of biochars obtained with other concentrations. Because the surface of most untreated biochars are neutral or alkaline pH, such materials usually have good cation exchange capacity and poor anion adsorption ability (Kameyama et al., 2016). Zirconium oxide/hydroxide compounds on the surface of biochar can supply active adsorption sites for anions, such as phosphate, fluoride, and sulfate (Cui et al., 2012; Su et al., 2013; Yu et al., 2018). A low initial concentration of ZCO resulted in the poor

sulfate adsorption (Fig. 1) which is due to the decrease in loading amount of zirconium oxides at a low initial concentration (Mulinari and da Silva, 2008; Robson, 2004; Zhao et al., 2011). A high concentration of initial ZCO also caused a decrease in sulfate adsorption (Fig. 1). Therefore, a moderate concentration (0.5 mol/L) of ZCO was selected to prepare ZrBC, which was used in subsequent experiments on the sulfate adsorption performance. 3.2 FE-SEM/EDS characterization SEM images of PP, PBC and ZrBC are presented in Fig. 2. As shown in Fig. 2a and Fig. 2d, PP has a honeycomb structure with many large pores inside, and probably consists of cellulose, hemicellulose, and lignin (Torab-Mostaedi et al., 2013). It can be clearly seen in Fig. 2b that the PBC surface is rougher than that of PP and many slice structures are apparent, resulting from pore collapse during the pyrolysis process of PP (Han et al., 2013). There are many agglomerates deposited across the entire matrix of ZrBC, in contrast to PBC (Fig. 2b-2c, Fig. 2e-2f), and the aggregated particles have diameters in the range of 0.2~5 μm. Furthermore, EDS element mapping analysis (Fig. S1) confirms that the aggregated particles contain substantial amount zirconium and oxygen, indicating that zirconium oxides were formed on the surface of PBC. 3.3 BET specific surface area and pore structure analysis Pore size distribution and pore structure of PP, PBC, and ZrBC were determined by N2 adsorption-desorption isotherms (Fig. 3), and information on the specific surface area, total pore volume and average pore radius are listed in Tab. 1. As shown in Fig. 3a, the shapes of adsorption-desorption isotherms are consistent with type III isotherm according to the IUPAC classification (Sing et al., 1985). The specific surface area and total pore volume of ZrBC are larger than those of the other materials, whereas average pore radius of ZrBC is smaller than that of PBC. This is possibly because loading

zirconium oxides on the PBC resulted in a very rough surface, which is in agreement with the FE-SEM/EDS analysis. ZrBC has more mesoporous than the other samples, as showed in Fig.3b, which also enhances the specific surface area and total pore volume of ZrBC. In addition, the pore structure of PP probably collapsed during pyrolysis process, which resulted in a decrease in surface area in PBC. The experimental results also show that the pyrolysis and chemical precipitation process did not contribute to the formation of micropores on biochar materials (Fig. 3b). 3.4 XPS characterization The XPS characterization results of PBC, ZrBC and post-adsorption ZrBC are shown in Fig. 4 and Tab. 2. Based on comparison with PBC, zirconium was successfully loaded on the surface of ZrBC (Fig. 4a), which is in accordance with the analysis results of FE-SEM/EDS. The weak peak found at 169.16 eV (Fig. 4b) was assigned to S 2p spectrum (Hong et al., 2014), which confirms the adsorption of sulfate ions on the ZrBC surface . The Zr 3d spectra of ZrBC and post-adsorption ZrBC were both fitted by two peaks, corresponding to the Zr3d5/2 and Zr3d3/2 electronic orbitals, respectively (Wang et al., 2019). After sulfate adsorption on ZrBC, the binding energies of the Zr3d peaks decreased by approximately 0.17 eV and 0.14 eV (Fig. 4c), which is attributed to the bonding between sulfate anion and zirconium (He et al., 2016; Yu et al., 2018). The O1s XPS spectra are consistent with two overlapping O1s peaks of Zr-OH and O-C, and the percentage of hydroxyl groups decreases from 21.19% to 3.24% after sulfate adsorption (Fig. 4d and Tab. 2), which indicates that hydroxyl groups (-OH) on the ZrBC surface participate in the adsorption of sulfate. Furthermore, the increase in solution pH observed after adsorption confirms the release of -OH from ZrBC during the removal process of sulfate. It can be inferred that the adsorption mechanism of sulfate anion on ZrBC is probably ligand exchange between -OH and sulfate anion based on Zr4+(Yu et al., 2018).The exchange formula can be described as equation (2).

ZrO(OH)2 + SO42- = ZrOSO4 + 2OH-

(2)

3.5 Effect of pH on the adsorption of sulfate on ZrBC The effect of the solution pH on sulfate adsorption by ZrBC is shown in Fig. 5. When the solution pH value increased from 2.0 to 12.0, the adsorption capacity decreased gradually from 34.26 mg/g to 1.31 mg/g. The sulfate adsorption capacity was highly dependent on the initial solution pH, and the optimum pH was approximately 2.0. The variation in sulfate adsorption capacity with pH can be attributed to the surface charge of the adsorbent. The point of zero charge (pHPZC) of ZrBC was determined to be 6.19, and detailed information is shown in Fig. S2. When the pH of the sulfate solution is lower than pHPZC, the ZrBC surface will be protonated and positively charged (Wang et al., 2016). Sulfate anions are easily captured by ZrBC through electrostatic attraction. Otherwise, the ZrBC surface is deprotonated at high pH values, resulting in an increase in electrostatic repulsion and a decrease in sulfate uptake by ZrBC. Moreover, the OH- ions that exist in solution in large numbers at high pH will compete with sulfate ions for the active adsorption sites on the ZrBC surface, which also leads to a lower sulfate adsorption capacity. 3.6 Adsorption isotherms and modeling analysis The isotherm behaviors of sulfate ions on PBC and ZrBC were examined, and the results are shown in Fig. 6. The sulfate adsorption amount of PBC was as low as 1.02 mg/g, while the adsorption amount of ZrBC reached 35.14 mg/g. The adsorption amount of ZrBC was obviously increased approximately 30-40 times compared with PBC under the same experimental conditions. In addition, five different isotherm models were used to analyze the isotherms data. The related equations are respectively shown as follows (Sun et al., 2015; Yao et al., 2011).

Qmax K l Ce , 1  K l Ce

(3)

Freundlich model: qe  K f (Ce ) ,

(4)

Langmuir model: qe 

n

Langmuir-Freundlich model: qe 

Redlich-Peterson model: qe  Temkin model: qe 

Qmax K lf (Ce ) n 1  K lf (Ce ) n

K r Ce , 1  aCe

RT ln( ACe ) , b

,

(5)

(6)

(7)

where qe (mg/g) and Ce (mg/L) are the adsorption capacity and sulfate concentration at equilibrium, respectively; Qmax (mg/g) represents the maximum adsorption capacity; Kl (L·mg-1), Kf (mg1-n·Ln·g-1), Klf (Ln·mg-n), and Kr (L·g-1) are the Langmuir, Freundlich, Langmuir-Freundlich and Redlich-Peterson constants, respectively; n (dimensionless) is the Freundlich linear constant; a (Ln·mg-n) is the Redlich-Peterson isotherm constant; A (L·mol-1) is the Temkin equilibrium binding constant and b (J·g·mg-1) is the Temkin constant (Sun et al., 2015; Yao et al., 2011). Except for Langmuir model, which is a theoretical equation, all the other models use empirical or semiempirical equations describe heterogeneous adsorption. The resulting parameters are summarized in Tab. 3. The Langmuir model matched the experimental data better than the other models from the comparison of correlation coefficient (R2) values, which indicated that monolayer adsorption probably occurred (Yao et al., 2013). This result is consistent with reported studies on sulfate adsorption in a rape straw biochar-soil system (Zhao et al., 2017) and on polypyrrole-modified activated carbons (Hong et al., 2017). The Qmax of ZrBC calculated from the Langmuir isotherm model is 35.21 mg/g. Moreover, the partition coefficient (PC), the ratio of the maximum adsorption amount to the equilibrium concentration of sulfate, is also an important parameter to evaluate the actual performance of an adsorbent in addition to the adsorption capacity (Gogoi et al., 2019; Na et al., 2019; Vikrant and Kim, 2019). The PC value for sulfate adsorption on ZrBC is 0.014 mg·g-1·μM-1 mg, which is near the values reported in other sulfate adsorption studies. A detailed

comparison between ZrBC and other sulfate adsorbents is shown in Tab. 4. Furthermore, RL was calculated according to equation (8) to confirm the favorability of sulfate adsorption on ZrBC (Sun et al., 2015).

RL 

1 , 1  K l C0

(8)

where Kl (L·mg-1) is the Langmuir constant and C0 (mg·L-1) is the initial solution concentration of sulfate. The RL values calculated with equation (8) are in the range of 0 to 1, suggesting that sulfate adsorption on ZrBC is favorable (Sun et al., 2015). 3.7 Adsorption kinetics The adsorption kinetics of sulfate ions on ZrBC at different temperatures were investigated and are shown in Fig.7. Fig.7a shows that the overall sulfate adsorption process can be divided into two stages: the rapid-increase stage and the near-equilibrium stage. The equilibrium time for sulfate adsorption was approximately 120 min. In addition, sulfate adsorption decreased with increasing of experimental temperature, indicating that sulfate adsorption was exothermic and that a lower temperature was beneficial to adsorption. This result agrees with previous reports on sulfate adsorption by Ni–Al and Ni–Fe quartz-albitophire (Sadeghalvad et al., 2016). To further explore the kinetic characteristics, the pseudo first- and second-order models were applied to fit the adsorption kinetic data (Azizian, 2004; Sadeghalvad et al., 2016). The kinetic equations are shown as follows.

dqt  k1 ( qe  qt ) , dt dq 2 Pseudo second-order equation: t  k2 ( qe  qt ) , dt Pseudo first-order equation:

(9) (10)

where qt (mg·g-1) and qe (mg·g-1) are the amounts of sulfate adsorbed at time t and at equilibrium, respectively; and k1 (min-1) and k2 (g· mg-1·min-1) are the rate constants for the pseudo first- and second-order models, respectively. The analysis results from pseudo first- and second-order models are demonstrated in Fig. 7b and Fig. 7c, respectively. The related parameters are listed in Tab. 5. The kinetics

of sulfate adsorption on the ZrBC are well described by the pseudo second-order model, whose correlation coefficient (R2) is closer to 1.0, and the calculated equilibrium adsorption amounts are much closer to the experimental

values.

A

previous

reported

study

also

claimed

that

sulfate

adsorption

on

polypyrrole-modified activated carbon follows a pseudo second-order models (Hong et al., 2017). As shown in Tab. 4, the pseudo second-order rate constant (k2) decreases with increasing of temperature from 288 K to 318 K, suggesting that a decrease in temperature may accelerate adsorption. 3.8 Competitive adsorption The effects of different coexisting anions on sulfate removal by ZrBC are shown in Fig. 8. The presence of PO43- decreases the adsorption of sulfate under the same concentration conditions. A previous study (Zhang et al., 2017) showed that zirconium oxide/hydroxide also has affinity toward phosphate, which can compete with sulfate for the active adsorption sites on ZrBC, resulting in a decrease in sulfate removal. For NO3- and F-, the decrease in sulfate adsorption is small, which is in accordance with a reported study (Zhang et al., 2017). It can be concluded that NO3- and F- have little influence on sulfate removal by ZrBC, whereas PO43- can impact it under the same concentration conditions. In the case of practical sulfate wastewater, such as acid mine drainage, interference from coexisting anions to sulfate removal will not be significant because the concentration of sulfate is much higher than that of other anions. 4 Conclusion In this study, a novel and low-cost adsorbent, ZrBC, was synthesized by a chemical precipitation method for sulfate removal from aqueous solution. The adsorbent has a high specific surface area and total pore volume and contains numerous zirconium oxides and hydroxyl groups. The sulfate adsorption isotherm agrees well with the Langmuir adsorption model. The maximum sulfate adsorption capacity of

ZrBC was calculated as 35.21 mg/g, which is much higher than that of the original PBC (1.02 mg/g). The adsorption of sulfate is pH dependent and better adsorption was achieved at acidic conditions. The adsorption equilibrium time is approximately 120 min and the adsorption process can be well described by the pseudo second-order equation. Sulfate adsorption is exothermic, and a lower temperature is beneficial to adsorption. In addition, nitrate and fluoride have little effect on sulfate removal, whereas phosphate can decrease adsorption under the same concentration conditions. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 51408239), the Natural Science Foundation of Fujian Province, China (No. 2016J01193), and the Fundamental Research Funds for the Central Universities (No. ZQN-712).

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Table and Figure List Tables Tab. 1 BET-N2 specific surface area (SA), total pore volume (TPV), and average pore radius (APR) of PP, PBC, and ZrBC. Tab. 2 XPS full-spectrum scanning data for PBC, ZrBC, and post-sorption ZrBC. Tab. 3 Adsorption isotherm model parameters and correlation coefficients (R2) for sulfate adsorption by ZrBC. Tab. 4 Comparison of ZrBC with other sulfate adsorbents Tab. 5 Adsorption kinetic model parameters and correlation coefficients (R2) for sulfate adsorption by ZrBC.

Figures Fig. 1 Effect of initial ZrOCl2 concentration on ZrBC for the adsorption of sulfate ion. Fig. 2 FE-SEM/EDS images of PP (a, d), PBC (b, e), and ZrBC (c, f). Fig. 3. N2 adsorption-desorption isotherms (a) and the pore size distributions (b) of PP, PBC, and ZrBC. Fig. 4 XPS spectra of PBC, ZrBC, and post-sorption ZrBC: (a) full-spectrum scanning; (b) S2p orbital spectrum for post-sorption ZrBC ; (c) Zr3d orbital and (d) O1s orbital spectra for ZrBC and post-sorption ZrBC. Fig. 5 Effect of pH on the sulfate adsorption capacity of ZrBC. Fig. 6 Adsorption isotherm of sulfate by PBC and ZrBC (a) and isotherm models of sulfate adsorption on ZrBC (b). Fig. 7 Adsorption kinetics of sulfate adsorption on ZrBC at different temperature: (a) raw data, (b) linear

fitting of the pseudo first-order kinetic model, and (c) linear fitting of the pseudo second-order kinetic model. Fig. 8 Effect of coexisting anions on sulfate removal by ZrBC. (The initial concentrations of sulfate ion and the added coexisting anion are both 1.0 mmol/L).

Tab. 1 BET-N2 specific surface area (SA), total pore volume (TPV), and average pore radius (APR) of PP, PBC, and ZrBC Adsorbent

SA(m2/g)

TPV(cm3/g)

APR(nm)

PP

10.3551

0.007772

3.00221

PBC

6.7238

0.010585

6.29709

ZrBC

21.7626

0.034591

4.8632

Tab. 2 XPS full-spectrum scanning data for PBC, ZrBC, and post-sorption ZrBC Atomic (%) biochar C

O

Zr

Cl

N

S

PBC

82.44

13.65

-

0.44

1.86

-

ZrBC

62.10

22.90

4.57

3.93

2.62

0.37

post-sorption ZrBC

69.14

22.08

2.64

1.27

2.37

0.78

Tab. 3 Adsorption isotherm model parameters and correlation coefficients (R2) for sulfate adsorption by ZrBC Parameter 3

R2

Parameter 1

Parameter 2

Langmuir

Kl=0.135

Qmax=35.208

0.923

Freundlich

Kf=8.494

n=0.277

0.789

Langmuir- Freundlich

Klf=0.077

Qmax=32.534

n=-0.557

0.922

Redlich-Peterson

Kr=4.692

a=0.129

n=1.006

0.904

Temkin

b=391.591

A=1.640

0.892

Tab. 4 Comparison of ZrBC with other sulfate adsorbents

absorbents

pH value

Initial concentration (mg·L-1)

Equilibrium concentration (mg·L-1)

Adsorption capacity (mg·g-1)

Partition coefficient (mg·g-1·μM-1)

Reference

Barium modified blast-furnace slag geopolymer

7-8

-

750

90.0

0.012

(Runtti et al., 2016)

Organo-nanoclay

7.0

-

300

20.0

0.0064

(Chen and Liu, 2014)

PG-Peat

2.4

1835

1050

189.5

0.017

(Gogoi et al., 2019)

Polypyrrole-grafted granular activated carbon

-

250

140

44.7

0.031

(Hong et al., 2014)

ZrBC

2

300

240

35.2

0.014

Present study

Tab. 5 Adsorption kinetic model parameters and correlation coefficients (R2) for sulfate adsorption by ZrBC qe,exp

pseudo first-order equation

pseudo second-order equation

T(K) (mg·g-1)

qe,Cal(mg·g-1)

k1(1·min-1)

R2

qe,Cal(mg·g-1)

k2(g· mg-1·min-1)

R2

288K

29.967

12.210

0.0177

0.949

30.026

0.0050

0.999

298K

28.016

8.488

0.0046

0.897

28.111

0.0026

0.998

308K

27.226

10.798

0.0049

0.789

27.393

0.0021

0.998

318K

18.766

8.767

0.0040

0.516

17.052

0.0024

0.939

Fig. 1 Effect of initial ZrOCl2 concentration on ZrBC for the adsorption of sulfate ion

a

b

PP (500×) d

c

PBC (500×) e

PP (5000×)

ZrBC (500×) f

PBC (5000×)

ZrBC (5000×)

Fig. 2 FE-SEM/EDS images of PP (a, d), PBC (b, e), and ZrBC (c, f)

a

b

Fig. 3 N2 adsorption-desorption isotherms (a) and the pore size distributions (b) of the PP, PBC, and ZrBC

a

b

c

d

Fig. 4 XPS spectra of PBC, ZrBC, and post-sorption ZrBC: (a) full-spectrum scanning; (b) S2p orbital spectrum for post-sorption ZrBC ; (c) Zr3d orbital and (d) O1s orbital spectra for ZrBC and post-sorption ZrBC

Fig. 5 Effect of pH on the sulfate adsorption capacity of ZrBC

Fig. 6 Adsorption isotherm of sulfate by PBC and ZrBC (a) and isotherm models of sulfate adsorption on ZrBC (b)

b

a

c

Fig. 7 Adsorption kinetics of sulfate adsorption on ZrBC at different temperature: (a) raw data, (b) linear fitting of the pseudo first-order kinetic model, and (c) linear fitting of the pseudo second-order kinetic model

Fig. 8 Effect of coexisting anions on sulfate removal by ZrBC. (The initial concentrations of sulfate ion and the added coexisting anion are both 1.0 mmol/L)

Highlights

1) Zr oxide-modification enhanced sulphate adsorption on PBC significantly. 2) XPS confirmed presence of Zr oxides and hydroxyl groups on ZrBC. 3) Acidic conditions favor sulphate removal by ZrBC. 4) The adsorption follows Langmuir model and pseudo second-order rate equation.